As disclosed in U.S. Pat. No. 2,297,691 to Carlson, an electrophotographic process employs a photosensitive material comprising a substrate which has been coated in a dark room with an insulating material which changes its electrical resistance depending on the amount of irradiation during imagewise exposure. Such a photosensitive material is generally given a uniform surface electrical charge after being adapted to darkness for a suitable period of time. The material is then exposed to a desired image by an irradiation pattern which has an effect of reducing surface electrical charge depending on relative energy contained in various portions of the irradiation pattern. The surface electrical charge or static latent image thus left behind on the surface of the photoconductive material layer (electrophotographic photosensitive layer) is then brought into contact with a suitable electroscopic displaying substance or toner to develop a visible image.
Such a toner may be contained in either an insulating liquid or dry carrier. In either case, the toner may be attached to the surface of an electrophotographic photosensitive layer in accordance with an electrical charge pattern. The displaying substance thus attached may be fixed to the layer by a known means such as heat, pressure, and solvent vapor. The static latent image may be transferred to a second substrate (e.g., paper and film). Accordingly, the static latent image may be developed on such a second substrate.
Principle requirements in an electrophotographic process include that (1) the photoconductive material can be charged with a desired potential in a dark room, (2) the dissipation of electrical charge in a dark room is negligibly small, and (3) the electrical charge can be rapidly dissipated upon light irradiation.
Heretofore, photoconductive materials for electrophotographic photosensitive material that have been employed include selenium, cadmium sulfide, and zinc oxide.
It is known that these inorganic materials have many advantages but, at the same time, have many disadvantages. For example, selenium, which is now widely used, satisfies the above requirements but is disadvantageous in that its complex production conditions entail high production costs. This material is also disadvantageous in that its poor flexibility makes it difficult to be worked into a belt-shaped form, and its high susceptibility to heat and mechanical impact requires careful handling. Cadmium sulfide or zinc oxide is dispersed in a binder such as a resin to be used as an electrophotographic photosensitive material. However, such an electrophotographic photosensitive material is disadvantageous in mechanical properties such as smoothness, rigidity, tensile strength, and abrasion resistance, and thus cannot sufficiently repeatedly be used in its heretofore known embodiments.
In recent years, electrophotographic photosensitive materials employing various organic materials have been proposed and put into practical use to eliminate these problems of inorganic materials. These electrophotographic photosensitive materials include an electrophotographic photosensitive material made of poly-N-vinylcarbazole and 2,4,7-trinitrofluorene (see U.S. Pat. No. 3,484,237), an electrophotographic photosensitive material which comprises poly-N-vinylcarbazole sensitized with a pyrylium salt dye (see Japanese patent publication No. 25658/73), an electrophotographic photosensitive material mainly comprising an organic pigment (see Japanese patent application (OPI) No. 37543/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")), and an electrophotographic photosensitive material mainly comprising an eutectic complex made of a dye and a resin (see Japanese patent application (OPI) No. 10735/72).
If a proper binder is selected, an electrophotographic photosensitive material employing such an organic material can be applied to a substrate by a coating method. Therefore, such an electrophotographic photosensitive material provides an extremely high productivity, providing an inexpensive photosensitive material. Such an electrophotographic photosensitive material has improved mechanical properties and flexibility. Furthermore, when a dye and an organic pigment are properly selected, the photosensitive wavelength can freely be controlled.
However, these electrophotographic photosensitive materials cannot fully meet requirements for electrophotographic photosensitive material because of various disadvantages. These disadvantages include that (1) they are low in photosensitivity, (2) the higher photosensitivity is, the poorer is chargeability, (3) the higher photosensitivity is, the higher is electrical charge dissipation in a dark room (dark decay), (4) when repeatedly used, they show large fluctuations in properties such as photosensitivity, chargeability, and dark decay, and (5) an inorganic pigment cannot be easily dispersed in the material, resulting in a poor coated surface.
Heretofore, the method for recording data on a data recording medium by irradiating high energy density beams thereonto so that the physical properties such as transmittance, reflectance, and refractive index thereof are changed has been used for COM (computer output micro), microfacsimile, printing plates, and optical discs, because it is advantageous in that it can produce images of extremely high resolution and contrast, enables addition of data, and can simultaneously record data upon exposure to light, thus making it possible to be used for recording of output of computers or time series signals transmitted.
For example, a recording medium used in an optical disc technique enables high density recording of data by recording pits of about 1 .mu.m in diameter in the form of a spiral or circular tracks. In order to write data into such a disc, a laser beam converged onto the surface of a laser-sensitive layer is scanned so that the surface thus irradiated forms pits. These pits are arranged in the form of spiral or circular tracks. In the heat mode recording process, a laser-sensitive layer absorbs heat energy and forms small indentations (pits) by evaporation or melting upon irradiation of laser beams.
The data thus recorded on the optical disc is detected by scanning laser beams along the track and reading optical changes caused by the difference between portions having pits and portions free of pits.
Heretofore, as such a data recording medium which enables heat mode recording, there have been employed a recording medium which comprises a transparent substrate such as plastic on which a thin film made of a metal and/or metal oxide semimetal dielectric substance or a thin film containing a self-oxidizing binder and a dye is provided as a recording layer, over which a protective layer is provided.
However, such prior art thin film mainly comprising an inorganic material is disadvantageous in that its high reflectance to laser beams entails a low efficiency of use of laser beams, making it difficult to obtain high sensitivity, or requiring a remarkably increased output of the laser beam for data recording.
On the other hand, the use of organic material is generally disadvantageous in that its absorption property becomes unstable toward longer wavelength regions, i.e., 600 nm or longer, and it is thus susceptible to decay by a slight change in temperature in this region.
Therefore, no recording medium has yet been developed which contains an organic thin film which fully satisfies various practical requirements for a recording medium having a capability for "direct read and write". These requirements include that its absorption efficiency to laser beams used must be high, that its reflectance is such that the focus control upon data reading can be properly made, and that various properties such as stability of recorded images are achieved.